Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A method for producing an exhaust gas purifying catalyst according to the
present invention includes step (a) of preparing a metal oxide support
containing zirconium; step (b) of preparing a solution containing
rhodium; and step (c) of adding the metal oxide support prepared in the
step (a), and ammonium carbonate, ammonium hydrogencarbonate or ammonia
water, to the solution prepared in the step (b) to obtain the solution
having a pH adjusted to a range of 3.0 or higher and 7.5 or lower. The
present invention provides a method capable of producing an exhaust gas
purifying catalyst including a metal oxide support containing zirconium
and rhodium of a minute particle size which is supported on the metal
oxide support at a high degree of dispersion.

Claims:

1. A method for producing an exhaust gas purifying catalyst, comprising:
step (a) of preparing a metal oxide support containing zirconium; step
(b) of preparing a solution containing rhodium; and step (c) of adding
the metal oxide support, and ammonium carbonate or ammonium
hydrogencarbonate or ammonia water, to the solution to obtain the
solution having a pH adjusted to a range of 3.0 or higher and 7.5 or
lower.

2. The method for producing an exhaust gas purifying catalyst of claim 1,
wherein the step (c) includes: step (c-1A) of mixing the metal oxide
support in the solution; and step (c-2A) of adding ammonium carbonate,
ammonium hydrogencarbonate or ammonia water to the solution after the
step (c-1A) to adjust the pH of the solution to a range of 3.0 or higher
and 7.5 or lower.

3. The method for producing an exhaust gas purifying catalyst of claim 1,
wherein the step (c) includes: step (c-1B) of adding ammonium carbonate,
ammonium hydrogencarbonate or ammonia water to the solution to adjust the
pH of the solution to a prescribed range; and step (c-2B) of mixing the
metal oxide support in the solution after the step (c-1B); wherein the
prescribed range in the step (c-1B) is set such that the pH of the
solution becomes a value in a range of 3.0 or higher and 7.5 or lower
after the step (c-2B) is performed.

4. The method for producing an exhaust gas purifying catalyst of claim 1,
wherein the pH of the solution obtained in the step (c) is in a range of
4.0 or higher and 6.5 or lower.

5. The method for producing an exhaust gas purifying catalyst of claim 1,
wherein the metal oxide support prepared in the step (a) contains
zirconium in a range of 50 mol % or higher and 95 mol % or lower as being
converted into an oxide.

6. The method for producing an exhaust gas purifying catalyst of claim 5,
wherein the metal oxide support prepared in the step (a) contains
zirconium in a range of 70 mol % or higher and 90 mol % or lower as being
converted into an oxide.

7. The method for producing an exhaust gas purifying catalyst of claim 1,
wherein the metal oxide support prepared in the step (a) contains at
least one metal material selected from the group consisting of cerium,
lanthanum and neodymium.

8. The method for producing an exhaust gas purifying catalyst of claim 1,
wherein the solution prepared in the step (b) has an absorbance of 0.8 or
less for a ray having a wavelength of 300 nm.

9. The method for producing an exhaust gas purifying catalyst of claim 1,
wherein the solution prepared in the step (b) has a chlorine content of
1000 ppm or less.

10. The method for producing an exhaust gas purifying catalyst of claim
1, further comprising step (d) of drying and burning the solution after
the step (c) to obtain catalyst powder containing the metal oxide support
and rhodium supported thereon.

11. The method for producing an exhaust gas purifying catalyst of claim
10, further comprising step (e) of forming a catalyst layer by use of the
catalyst powder on a surface of a honeycomb-like substrate.

12. A motor vehicle, comprising: an internal combustion engine; an
exhaust pipe for guiding exhaust gas from the internal combustion engine
to outside; and the exhaust gas purifying catalyst produced by the method
for producing an exhaust gas purifying catalyst of claim 1 and provided
in the exhaust pipe.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a method for producing an exhaust
gas purifying catalyst, and specifically to a method for producing an
exhaust gas purifying catalyst including a metal oxide support containing
zirconium and rhodium supported thereon. The present invention also
relates to a motor vehicle including an exhaust gas purifying catalyst
produced by such a production method.

BACKGROUND ART

[0002] In order to purify combustion gas (exhaust gas) discharged from an
internal combustion engine of a motor vehicle, three-way catalysts are
widely used. A three-way catalyst reduces or oxidizes CO (carbon
monoxide), HC (hydrocarbon) and NOx (oxide of nitrogen) contained in
exhaust gas into water, carbon dioxide and nitrogen to purity the exhaust
gas. Such a three-way catalyst includes a support formed of a metal oxide
(metal oxide support), and a noble metal material such as platinum (Pt),
rhodium (Rh), palladium (Pd) or the like supported on the support.

[0003] As the metal oxide support, it is conventionally common to use
alumina (Al2O3) in order to provide a relatively large specific
surface area. However, it has recently been proposed to use a metal oxide
other than alumina such as ceria (CeO2), zirconia (ZrO2),
titania (TiO2) or the like in stead of, or in combination with,
alumina in order to utilize chemical characteristics of the metal oxide
support to further improve the purification performance.

[0004] Studies are also made on preferable combinations (chemistry) of a
metal oxide support and a noble metal material. It has been reported that
rhodium, when used in combination with a metal oxide containing zirconia
as a main component (zirconia or zirconia-based complex oxide), provides
superb purification performance.

[0005] When rhodium is supported by alumina, which is conventionally used
commonly, rhodium is dissolved into alumina as time passes, and this
decreases the catalyst activity. By contrast, when rhodium is supported
by a metal oxide containing zirconia as a main component, rhodium is not
dissolved. Therefore, the high catalyst activity which rhodium originally
has can be utilized.

[0006] However, it is difficult to cause a metal oxide containing zirconia
as a main component to adsorb and thus support rhodium. When a metal
oxide containing zirconia as a main component is merely mixed in a
commercially available aqueous solution of rhodium, rhodium is not
adsorbed to the metal oxide support almost at all.

[0007] Patent Documents 1 and 2 each disclose a technique for causing a
metal oxide containing zirconia as a main component to support rhodium.

[0008] According to the technique disclosed in Patent Document 1, zirconia
is added to an aqueous solution of rhodium nitrate and the resultant
solution is evaporated to dryness, so as to cause zirconia to support
rhodium. According to the technique disclosed in Patent Document 2, a
metal oxide containing zirconia as a main component is immersed in a
colloidal solution containing rhodium, so as to cause the metal oxide
support to support colloidal rhodium.

[0011] However, with the technique of Patent Document 1, rhodium
aggregates during evaporation to dryness, which decreases the degree of
dispersion of rhodium. With the technique of Patent Document 2, it is
difficult to decrease the size of the supported rhodium particles to a
sufficiently small level because it is difficult to make colloidal
particles of rhodium minute. When the degree of dispersion of rhodium is
low or the size of the rhodium particles is large, the number of rhodium
atoms actually contacting exhaust gas on a surface of the catalyst is
small, and therefore, a sufficiently high level of purification
performance cannot be provided.

[0012] The present invention made in light of this problem has an object
of providing a method capable of producing an exhaust gas purifying
catalyst including a metal oxide support containing zirconium (Zr) and
rhodium of a minute particle size which is supported on the metal oxide
support at a high degree of dispersion.

Solution to Problem

[0013] A method for producing an exhaust gas purifying catalyst according
to the present invention includes step (a) of preparing a metal oxide
support containing zirconium; step (b) of preparing a solution containing
rhodium; and step (c) of adding the metal oxide support, and ammonium
carbonate or ammonium hydrogencarbonate or ammonia water, to the solution
to obtain the solution having a pH adjusted to a range of 3.0 or higher
and 7.5 or lower.

[0014] In a preferable embodiment, the step (c) includes step (c-1A) of
mixing the metal oxide support in the solution; and step (c-2A) of adding
ammonium carbonate, ammonium hydrogencarbonate or ammonia water to the
solution after the step (c-1A) to adjust the pH of the solution to a
range of 3.0 or higher and 7.5 or lower.

[0015] In a preferable embodiment, the step (c) includes step (c-1B) of
adding ammonium carbonate, ammonium hydrogencarbonate or ammonia water to
the solution to adjust the pH of the solution to a prescribed range; and
step (c-2B) of mixing the metal oxide support in the solution after the
step (c-1B). The prescribed range in the step (c-1B) is set such that the
pH of the solution becomes a value in a range of 3.0 or higher and 7.5 or
lower after the step (c-2B) is performed.

[0016] In a preferable embodiment, the pH of the solution obtained in the
step (c) is in a range of 4.0 or higher and 6.5 or lower.

[0017] In a preferable embodiment, the metal oxide support prepared in the
step (a) contains zirconium in a range of 50 mol % or higher and 95 mol %
or lower as being converted into an oxide.

[0018] In a preferable embodiment, the metal oxide support prepared in the
step (a) contains zirconium in a range of 70 mol % or higher and 90 mol %
or lower as being converted into an oxide.

[0019] In a preferable embodiment, the metal oxide support prepared in the
step (a) contains at least one metal material selected from the group
consisting of cerium, lanthanum and neodymium.

[0020] In a preferable embodiment, the solution prepared in the step (b)
has an absorbance of 0.8 or less for a ray having a wavelength of 300 nm.

[0021] In a preferable embodiment, the solution prepared in the step (b)
has a chlorine content of 1000 ppm or less.

[0022] In a preferable embodiment, the method for producing an exhaust gas
purifying catalyst according to the present invention further includes
step (d) of drying and burning the solution after the step (c) to obtain
catalyst powder containing the metal oxide support and rhodium supported
thereon.

[0023] In a preferable embodiment, the method for producing an exhaust gas
purifying catalyst according to the present invention further includes
step (e) of forming a catalyst layer by use of the catalyst powder on a
surface of a honeycomb-like substrate.

[0024] A motor vehicle according to the present invention includes an
internal combustion engine; an exhaust pipe for guiding exhaust gas from
the internal combustion engine to outside; and the exhaust gas purifying
catalyst produced by the above-described method for producing an exhaust
gas purifying catalyst and provided in the exhaust pipe.

[0025] Hereinafter, the function of the present invention will be
described.

[0026] The method for producing an exhaust gas purifying catalyst
according to the present invention includes step (a) of preparing a metal
oxide support containing zirconium; and step (b) of preparing a solution
containing rhodium. The production method according to the present
invention further includes step (c) of adding the metal oxide support
prepared in the step (a), and ammonium carbonate, ammonium
hydrogencarbonate or ammonia water, to the solution prepared in the step
(b) to obtain the solution having a pH adjusted to a range of 3.0 or
higher and 7.5 or lower. When the pH of the solution obtained in the step
(c) is in a range of 3.0 or higher and 7.5 or lower, the degree of
adsorption of rhodium to the metal oxide support containing zirconium can
be increased. Therefore, it is not necessary to cause rhodium which has
not been adsorbed to be supported on the metal oxide support
semi-forcibly by evaporation to dryness or to make rhodium colloidal.
Thus, rhodium of a minute particle size can be supported on the metal
oxide support containing zirconium at a high degree of dispersion. For
this reason, according to the present invention, the level of
purification performance of the exhaust gas purifying catalyst can be
raised. In addition, in the production method according to the present
invention, ammonium carbonate, ammonium hydrogencarbonate or ammonia
water is used instead of a mere alkaline compound. Owing to this, the
degree of dispersion of rhodium after the catalyst is exposed to a high
temperature can be kept higher than in the case where any other alkaline
compound is used. Therefore, the catalyst produced by the production
method according to the present invention is highly durable and thus is
preferably usable as an exhaust gas purifying catalyst, which is exposed
to high-temperature exhaust gas.

[0027] The step (c) includes, for example, step (c-1A) of mixing the metal
oxide support in the solution; and step (c-2A) of adding ammonium
carbonate, ammonium hydrogencarbonate or ammonia water to the solution
after the step (c-1A) to adjust the pH of the solution to a range of 3.0
or higher and 7.5 or lower. When the pH is adjusted after the metal oxide
support is mixed in the solution in this manner, there is an advantage
that rhodium can be dispersed more uniformly.

[0028] Alternatively, the step (c) includes step (c-1B) of adding ammonium
carbonate, ammonium hydrogencarbonate or ammonia water to the solution to
adjust the pH of the solution to a prescribed range; and step (c-2B) of
mixing the metal oxide support in the solution after the step (c-1B). In
this case, the prescribed range in the step (c-1B) is set such that the
pH of the solution becomes a value in a range of 3.0 or higher and 7.5 or
lower after the step (c-2B) is performed. When the pH is adjusted before
the metal oxide support is mixed in the solution in this manner, there is
an advantage that a metal oxide support containing a component which is
easily soluble in acid can be used.

[0029] It is preferable that the pH of the solution obtained in the step
(c) is in a range of 4.0 or higher and 6.5 or lower. When the pH of the
solution is in a range of 4.0 or higher and 6.5 or lower, the degree of
adsorption of rhodium can be further increased.

[0030] In order to allow the catalyst activity of rhodium to be exhibited
sufficiently, it is preferable that the metal oxide support prepared in
the step (a) contains zirconium in a range of 50 mol % or higher and 95
mol % or lower as being converted into an oxide. Namely, it is preferable
that the ratio of zirconia with respect to the metal oxide support is in
a range of 50 mol % or higher and 95 mol % or lower. When the ratio of
zirconia with respect to the metal oxide support is in a range of 50 mol
% or higher and 95 mol % or lower, the NOx purification ratio
realized by the resultant catalyst can be raised.

[0031] In order to allow the catalyst activity of rhodium to be exhibited
sufficiently, it is more preferable that the metal oxide support prepared
in the step (a) contains zirconium in a range of 70 mol % or higher and
90 mol % or lower as being converted into an oxide. Namely, it is more
preferable that the ratio of zirconia with respect to the metal oxide
support is in a range of 70 mol % or higher and 90 mol % or lower. When
the ratio of zirconia with respect to the metal oxide support is in a
range of 70 mol % or higher and 90 mol % or lower, the NOx
purification ratio realized by the resultant catalyst can be further
raised.

[0032] It is preferable that the metal oxide support prepared in the step
(a) contains at least one metal material selected from the group
consisting of cerium, lanthanum and neodymium. Namely, it is preferable
that the metal oxide support is a zirconia-based complex oxide than being
formed only of zirconia. When the metal oxide support contains cerium,
the metal oxide support can absorb oxygen in the atmosphere. This raises
the NOx purification ratio. When the metal oxide support contains
lanthanum, the surface area of the complex oxide is increased. This
raises the NOx purification ratio. When the metal oxide support
contains neodymium, the aggregation of rhodium can be suppressed. This
raises the NOx purification ratio.

[0033] It is preferable that the solution prepared in the step (b) has an
absorbance of 0.8 or less for a ray having a wavelength of 300 nm.
According to the studies performed by the present inventors, when the
absorbance for the ray having a wavelength of 300 nm is 0.8 or less, the
degree of dispersion of rhodium can be increased as compared with when
the absorbance for the ray having a wavelength of 300 nm exceeds 0.8. A
conceivable reason for this is that the state of rhodium ions in the
solution influences the degree of dispersion.

[0034] It is preferable that the solution prepared in the step (b) has a
chlorine content of 1000 ppm or less. Chlorine causes the catalyst to be
poisoned. Therefore, when the chlorine content of the solution exceeds
1000 ppm, a step of removing chlorine is required after rhodium is
supported. By preparing a solution having a chlorine content of 1000 ppm
or less, such a step becomes unnecessary. Thus, the production cost can
be decreased and the time duration required for the production can be
shortened.

[0035] The method for producing an exhaust gas purifying catalyst
according to the present invention, typically, further includes step (d)
of drying and burning the solution after the step (c) to obtain catalyst
powder containing the metal oxide support and rhodium supported thereon.
In the production method according to the present invention, the pH of
the solution obtained in the step (c) is in a range of 3.0 or higher and
7.5 or lower. Therefore, in the catalyst powder obtained in the step (d),
rhodium of a minute particle size can be supported on the metal oxide
support at a high degree of dispersion.

[0036] The method for producing an exhaust gas purifying catalyst
according to the present invention, typically, further includes step (e)
of forming a catalyst layer by use of the catalyst powder on a surface of
a honeycomb-like substrate. A honeycomb-like substrate has a large
specific surface area. Therefore, by forming the catalyst layer on the
surface of the honeycomb-like substrate, a surface area in which the
exhaust gas and rhodium contact each other can be increased, and thus the
exhaust gas purifying catalyst functions in a preferable manner.

[0037] A motor vehicle according to the present invention includes the
exhaust gas purifying catalyst produced by the production method
according to the present invention, and providing a high level of
purification performance and having a high durability, and therefore can
decrease the emission of NOx or the like.

Advantageous Effects of Invention

[0038] The present invention provides a method capable of producing an
exhaust gas purifying catalyst including a metal oxide support containing
zirconium and rhodium of a minute particle size which is supported on the
metal oxide support at a high degree of dispersion.

BRIEF DESCRIPTION OF DRAWINGS

[0039] FIG. 1 is a flowchart of a method for producing an exhaust gas
purifying catalyst in a preferable embodiment according to the present
invention.

[0040] FIG. 2 is a flowchart of a method for producing an exhaust gas
purifying catalyst in a preferable embodiment according to the present
invention.

[0041] FIG. 3 is a graph showing the concentration of remaining rhodium
when a metal oxide support is mixed in a rhodium solution and any of
various compounds is added in a prescribed amount.

[0042] FIG. 4 is a graph showing the relationship between the pH of the
rhodium solution in which the metal oxide support is mixed and the degree
of adsorption of rhodium.

[0043] FIG. 5 is a graph showing the degree of dispersion of rhodium in
the exhaust gas purifying catalysts in Examples 1 through 5 and
Comparative examples 1 through 3, after high-temperature heating.

[0044]FIG. 6(a) is a graph showing the absorbance (Abs.) of the rhodium
solution used in Examples 1 through 4, and FIG. 6(b) is a graph showing
the absorbance (Abs.) of the rhodium solution used in Example 5.

[0045] FIG. 7 is a graph showing the relationship between the ratio of
zirconia (mol %) with respect to the metal oxide support and the NOx
emission (g/km).

[0046] FIG. 8 is a flowchart of a method for producing an exhaust gas
purifying catalyst in a preferable embodiment according to the present
invention.

[0047] FIG. 9 is a side view schematically showing a motorcycle 100
including the exhaust gas purifying catalyst produced by the production
method in a preferable embodiment according to the present invention.

DESCRIPTION OF EMBODIMENTS

[0048] Hereinafter, embodiments of the present invention will be described
with reference to the drawings. The present invention is not limited to
the following embodiment.

[0049] First, with reference to FIG. 1, a method for producing an exhaust
gas purifying catalyst in this embodiment will be described. FIG. 1 is a
flowchart of a production method in this embodiment.

[0051] Next, a solution containing rhodium (Rh) (rhodium solution) is
prepared (step S2). The rhodium solution prepared in step S2 is acidic
(i.e., having a pH less than 7), and is typically an aqueous solution of
a rhodium salt. Examples of the aqueous solution of a rhodium salt
include an aqueous solution of rhodium nitrate and an aqueous solution of
hexaammine rhodium. In FIG. 1, step S2 of preparing a rhodium solution is
performed after step S1 of preparing a metal oxide support. The order of
step S1 and step S2 is not limited to this. Step S1 and step S2 may be
performed in any order.

[0052] Next, the metal oxide support is mixed in the rhodium solution
(step S3). For example, powder of the metal oxide support is added to the
rhodium solution.

[0053] Next, the pH of the rhodium solution in which the metal oxide
support is mixed is adjusted to a range of 3.0 or higher and 7.5 or lower
(step S4). Step S4 is specifically performed by adding ammonium
carbonate, ammonium hydrogencarbonate or ammonia water to the rhodium
solution. Typically, the rhodium solution, after being mixed with the
above-mentioned alkaline compound, is stirred by a stirrer or the like,
and then left at a prescribed temperature (e.g., 60° C.) for a
prescribed time duration (e.g., 1 hour to 5 hours). As a result of step
S4, rhodium is adsorbed to the metal oxide support containing zirconium.

[0054] Next, the rhodium solution is dried and burned (step S5). As a
result, catalyst powder containing the metal oxide support and rhodium
supported thereon is obtained. The drying operation is performed, for
example, at 120° C. for 300 minutes. The burning operation is
performed, for example, at 600° C. for 60 minutes.

[0055] Then, a catalyst layer is formed by use of the catalyst powder on a
surface of a honeycomb-like substrate (step S6). The substrate is formed
of a heat-resistant material such as a metal or ceramic material. The
substrate has therein a great number of cells defined by ribs. The
catalyst layer is formed on the surface of the substrate as follows.
First, the catalyst powder is mixed with a binder and water, and the
resultant mixture is pulverized to form a slurry. The binder is added in
order to prevent the catalyst layer from being delaminated from the
substrate. Materials usable as the binder include boehmite (hydrate of
alumina) and aluminum nitrate. In the process of forming the slurry, it
is preferable to adjust the pH of the slurry to a range of 3 to 5 in
order to stabilize the slurry. Next, the slurry is applied to the surface
of the substrate, and then dried and burned. In this manner, an exhaust
gas purifying catalyst can be produced.

[0056] As described above, the production method in this embodiment
includes step S4 of adding ammonium carbonate, ammonium hydrogencarbonate
or ammonia water to the rhodium solution in which the metal oxide support
is mixed and thus adjusting the pH of the rhodium solution to a range of
3.0 or higher and 7.5 or lower. Since the pH of the rhodium solution is
adjusted to a range of 3.0 or higher and 7.5 or lower in step S4, the
degree of adsorption of rhodium to the metal oxide support containing
zirconium can be increased as shown by inspection results later.
Therefore, it is not necessary to cause rhodium which has not been
adsorbed to be supported on the metal oxide support semi-forcibly by
evaporation to dryness as described in Patent Document 1. It is not
necessary either to make rhodium colloidal as described in Patent
Document 2. Thus, rhodium of a minute particle size can be supported on
the metal oxide support containing zirconium at a high degree of
dispersion. For this reason, according to the production method in this
embodiment, the level of purification performance of the exhaust gas
purifying catalyst can be raised.

[0057] According to the production method in this embodiment, in step S4,
ammonium carbonate, ammonium hydrogencarbonate or ammonia water is used
instead of a mere alkaline compound. Owing to this, as shown by
inspection results later, the degree of dispersion of rhodium after the
catalyst is exposed to a high temperature can be kept higher than in the
case where any other alkaline compound is used. Therefore, the catalyst
produced by the production method in this embodiment is highly durable
and thus is preferably usable as an exhaust gas purifying catalyst, which
is exposed to high-temperature exhaust gas.

[0058] Ammonium carbonate and ammonium hydrogencarbonate are easier to
handle than ammonia water. Therefore, in step S4, it is preferable to add
ammonium carbonate or ammonium hydrogencarbonate to the rhodium solution.

[0059] In step S4, it is preferable that the pH of the rhodium solution is
adjusted to a range of 4.0 or higher and 6.5 or lower. When the pH of the
rhodium solution is adjusted to a range of 4.0 or higher and 6.5 or
lower, the degree of adsorption of rhodium can be further increased.

[0060] In order to allow the catalyst activity of rhodium to be exhibited
sufficiently, it is preferable that the metal oxide support prepared in
step S1 contains zirconium in a range of 50 mol % or higher and 95 mol %
or lower as being converted into an oxide. Namely, it is preferable that
the ratio of zirconia with respect to the metal oxide support is in a
range of 50 mol % or higher and 95 mol % or lower. When the ratio of
zirconia with respect to the metal oxide support is in a range of 50 mol
% or higher and 95 mol % or lower, the NOx purification ratio
realized by the resultant catalyst can be raised.

[0061] In order to allow the catalyst activity of rhodium to be exhibited
sufficiently, it is more preferable that the metal oxide support prepared
in step S1 contains zirconium in a range of 70 mol % or higher and 90 mol
% or lower as being converted into an oxide. Namely, it is preferable
that the ratio of zirconia with respect to the metal oxide support is in
a range of 70 mol % or higher and 90 mol % or lower. When the ratio of
zirconia with respect to the metal oxide support is in a range of 70 mol
% or higher and 90 mol % or lower, the NOx purification ratio
realized by the resultant catalyst can be further raised.

[0062] It is preferable that the metal oxide support prepared in step S1
contains at least one metal material selected from the group consisting
of cerium (Ce), lanthanum (La) and neodymium (Nd). Namely, it is more
preferable that the metal oxide support is a zirconia-based complex oxide
than being formed only of zirconia.

[0063] When the metal oxide support contains cerium, the metal oxide
support can absorb oxygen in the atmosphere. This raises the NOx
purification ratio. When the metal oxide support contains lanthanum, the
surface area of the complex oxide is increased. This raises the NOx
purification ratio. When the metal oxide support contains neodymium, the
aggregation of rhodium can be suppressed. This raises the NOx
purification ratio.

[0064] It is preferable that the rhodium solution prepared in step S2 has
an absorbance of 0.8 or less for a ray having a wavelength of 300 nm.
According to the studies performed by the present inventors, when the
absorbance for the ray having a wavelength of 300 nm is 0.8 or less, the
degree of dispersion of rhodium can be increased than when the absorbance
for the ray having a wavelength of 300 nm exceeds 0.8. A reason for this
will be described in detail later.

[0065] It is preferable that the rhodium solution prepared in step S2 has
a chlorine content of 1000 ppm or less. Chlorine causes the catalyst to
be poisoned. Therefore, when the chlorine content of the rhodium solution
exceeds 1000 ppm, a step of removing chlorine is required after rhodium
is supported. By preparing a rhodium solution having a chlorine content
of 1000 ppm or less, such a step becomes unnecessary. Thus, the
production cost can be decreased and the time duration required for the
production can be shortened.

[0066] According to the production method in this embodiment, the pH of
the rhodium solution is adjusted to a range of 3.0 or higher and 7.5 or
lower in step S4. Therefore, in the catalyst powder obtained in step S5,
rhodium of a minute particle size can be supported on the metal oxide
support at a high degree of dispersion. The degree of dispersion of
rhodium is defined as a ratio of the number of rhodium atoms exposed to
the surface with respect to the total number of rhodium atoms. The degree
of dispersion of rhodium can be measured by, for example, a CO pulse
method. In order to realize a sufficiently high level of purification
performance, the degree of dispersion of rhodium in the catalyst powder
is preferably 70% or higher, and more preferably 80% or higher.

[0067] The production method shown in FIG. 1 uses only rhodium as the
noble metal material. Alternatively, another noble metal material may be
used in addition to rhodium. In order to purify all of CO, HC and
NOx at a high efficiency, it is preferable to use platinum and
palladium in addition to rhodium. FIG. 2 shows a flowchart of a
production method using rhodium, platinum and palladium.

[0068] In this case, as shown in FIG. 2, separately from a series of steps
S1 through S5 for obtaining catalyst powder containing a metal oxide
support and rhodium supported thereon, a series of steps S7 through S10
for obtaining catalyst powder containing a metal oxide support and
platinum supported thereon, and a series of steps S11 through S14 for
obtaining catalyst powder containing a metal oxide support and palladium
supported thereon are performed.

[0070] First, a metal oxide support is prepared (step S7). The metal oxide
support prepared in step S7 is, for example, an alumina-based oxide or a
ceria-based oxide.

[0071] Next, a solution containing platinum (platinum solution) is
prepared (step S8). The platinum solution prepared in step S8 is
typically an aqueous solution of a platinum salt. Examples of the aqueous
solution of a platinum salt include an aqueous solution of
dinitrodiammine platinum and an aqueous solution of hexaammine platinum.
Step S7 and step S8 may be performed in any order.

[0072] Next, the metal oxide support is mixed in the platinum solution
(step S9). Powder of the metal oxide support is added to the platinum
solution, or the metal oxide support dispersed in water beforehand is
added, together with water, to the platinum solution. Typically, after
this process, the platinum solution is stirred by a stirrer or the like
and then is left at a prescribed temperature for a prescribed time
duration. As a result of step S9, platinum is adsorbed to the metal oxide
support.

[0073] Next, the platinum solution is dried and burned (step S10). As a
result, catalyst powder containing the metal oxide support and platinum
supported thereon is obtained. The drying operation is performed, for
example, at 120° C. for 300 minutes. The burning operation is
performed, for example, at 600° C. for 60 minutes.

[0075] First, a metal oxide support is prepared (step S11). The metal
oxide support prepared in step S11 is, for example, an alumina-based
oxide or a ceria-based oxide.

[0076] Next, a solution containing palladium (palladium solution) is
prepared (step S12). The palladium solution prepared in step S12 is
typically an aqueous solution of a palladium salt. Examples of the
aqueous solution of a palladium salt include an aqueous solution of
palladium nitrate and an aqueous solution of dinitrodiammine palladium.
Step S11 and step S12 may be performed in any order.

[0077] Next, the metal oxide support is mixed in the palladium solution
(step S13). Powder of the metal oxide support is added to the palladium
solution, or the metal oxide support dispersed in water beforehand is
added, together with water, to the palladium solution. Typically, after
this process, the palladium solution is stirred by a stirrer or the like
and then is left at a prescribed temperature for a prescribed time
duration. As a result of step S13, palladium is adsorbed to the metal
oxide support.

[0078] Next, the palladium solution is dried and burned (step S14). As a
result, catalyst powder containing the metal oxide support and palladium
supported thereon is obtained. The drying operation is performed, for
example, at 120° C. for 300 minutes. The burning operation is
performed, for example, at 600° C. for 60 minutes.

[0079] After the catalyst powder containing rhodium, the catalyst powder
containing platinum, and the catalyst powder containing palladium are
obtained as described above, these types of catalyst powder may be used
to form a catalyst layer on a surface of a honeycomb-like substrate (step
S6). For example, first, the catalyst powder containing rhodium, the
catalyst powder containing platinum, the catalyst powder containing
palladium, a binder and water are mixed together, and the resultant
mixture is pulverized to form a slurry. In this process, another metal
oxide (e.g., alumina) may be added to the mixture in order to stabilize
the slurry. In the process of forming the slurry, it is preferable to
adjust the pH of the slurry to a range of 3 to 5 for the reason described
above. Next, the slurry is applied to the surface of the substrate, and
then dried and burned. In this manner, an exhaust gas purifying catalyst
containing rhodium, platinum and palladium as noble metal materials can
be produced.

[0080] As already described, since the pH of the rhodium is adjusted to a
range of 3.0 or higher and 7.5 or lower in step S4, the degree of
adsorption of rhodium to the metal oxide support containing zirconium can
be increased. Since ammonium carbonate, ammonium hydrogencarbonate or
ammonia water is used in step S4 instead of a mere alkaline compound, the
degree of dispersion of rhodium after the catalyst is exposed to a high
temperature can be kept higher than in the case where any other alkaline
compound is used. Hereinafter, results of inspection on these points will
be described.

[0081] FIG. 3 shows the concentration of remaining rhodium (concentration
of rhodium which is not adsorbed to the metal oxide support and remaining
in the solution) when a metal oxide support is mixed in a rhodium
solution and any of various compounds is added in a prescribed amount.
Specifically, FIG. 3 shows the concentration of remaining rhodium when a
zircoania-based complex oxide containing zirconia (ZrO2) and ceria
(CeO2) as well as a small amount of lanthania (La2O3) and
a small amount of neodymia (Nd2O3) was mixed in an aqueous
solution of rhodium nitrate, then any of various compounds was added, and
the resultant substance was left at 80° C. for 1 hour. The
concentration of remaining rhodium was measured by ICP (inductively
coupled plasma) emission analysis. Among the added compounds, ammonium
carbonate was added in twice the amount, three times the amount, five
times the amount, and ten times the amount (in the figure, represented as
"×2", "×3", "×5", "×10", respectively).

[0082] As can be seen from FIG. 3, when an alkaline compound such as
ammonium carbonate, ammonium hydrogencarbonate, ammonium water, sodium
hydroxide, potassium hydroxide or the like is added, the concentration of
remaining rhodium is lower than in the case where no such compound is
added; namely, the amount of rhodium adsorbed to the metal oxide support
is increased. By contrast, when an acidic compound such as citric acid,
oxalic acid, ethanol, acetic acid or the like is added, the concentration
of remaining rhodium is approximately equal to, or higher than, in the
case where no such compound is added; namely, the amount of rhodium
adsorbed to the metal oxide support is not changed almost at all or is
decreased.

[0083] FIG. 4 shows the relationship between the pH of the rhodium
solution in which the metal oxide support is mixed and the degree of
adsorption of rhodium. Specifically, FIG. 4 shows the relationship
between the pH and the degree of adsorption when the same zirconia-based
complex oxide as shown in FIG. 3 was mixed in an aqueous solution of
rhodium nitrate and kept at 80° C. The pH was adjusted by changing
the amount of ammonium carbonate added to the solution. The degree of
adsorption was calculated from the concentration of remaining rhodium.

[0084] When a zirconia-based complex oxide is merely mixed in a
commercially available aqueous solution of rhodium nitrate (i.e., without
pH adjustment), the pH is about 1.2. In this case, as can be seen from
FIG. 4, almost no amount of rhodium is adsorbed to the metal oxide
support. As disclosed in Patent Document 1, the solution could be
evaporated to dryness so that rhodium is forcibly supported on the metal
oxide support. However, when the solvent is evaporated in the state where
rhodium is not adsorbed to the metal oxide support, rhodium aggregates in
the process. This decreases the degree of dispersion.

[0085] By contrast, as can be seen from FIG. 4, when the pH is in a range
of 3.0 or higher and 7.5 or lower, a degree of adsorption of about 80% or
higher can be realized. Therefore, rhodium can be supported on the metal
oxide support containing zirconium at a high degree of dispersion. As can
be seen from FIG. 4, when the pH is in a range of 4.0 or higher and 6.5
or lower, a degree of adsorption of about 97% or higher can be realized.
Therefore, rhodium can be supported on the metal oxide support containing
zirconium at a higher degree of dispersion.

[0086] Table 1 shows the degree of dispersion of rhodium in an initial
state and the degree of dispersion of rhodium after high-temperature
heating (heating at 800° C. for five hours) in Examples 1 through
5 and Comparative examples 1 through 3. In Examples 1 through 5, ammonium
carbonate, ammonium hydrogencarbonate or ammonia water was added as an
alkaline compound to a rhodium solution in which the metal oxide support
is mixed. In Comparative example 1, no alkaline compound was added. In
Comparative examples 2 and 3, sodium hydroxide and potassium hydroxide
were added as an alkaline compound respectively. Even in the initial
state, the metal oxide support supporting rhodium has already been heated
at 600° C. for 1 hour in the process of burning the slurry. The
degree of dispersion was measured by a CO pulse method. Table 1 also
shows the absorbance (Abs.) of the rhodium solution prepared in step S2
for the ray having a wavelength of 300 nm, and the ratio of zirconia (mol
%) with respect to the metal oxide support prepared in step S1. The
degree of dispersion of rhodium after at a high-temperature heating shown
in Table 1 is shown in FIG. 5 in a graph.

[0087] As seen in Table 1, the degree of dispersion of rhodium in the
initial state is higher in Examples 1 through and Comparative examples 2
and 3 than in Comparative example 1. This occurs because the degree of
adsorption of rhodium to the metal oxide support is increased by addition
of an alkaline compound.

[0088] As shown in Table 1 and FIG. 5, the degree of dispersion of rhodium
after high-temperature heating is higher in Examples 1 through 5 than in
Comparative examples 1 through 3. This occurs because use of ammonium
carbonate, ammonium hydrogencarbonate or ammonia water as an alkaline
compound allows the degree of dispersion of rhodium to be kept high even
if the catalyst is exposed to a high temperature. A conceivable reason
why the degree of dispersion of rhodium is significantly decreased after
high-temperature heating when sodium hydroxide or potassium hydroxide is
used as an alkaline compound is that sodium or potassium reacts with the
catalyst component at a high temperature.

[0089] In Examples 1 through 4, and in Example 5, different rhodium
solutions were used. FIG. 6(a) shows the absorbance (Abs.) of the rhodium
solution used in Examples 1 through 4. FIG. 6(b) shows the absorbance
(Abs.) of the rhodium solution used in Example 5. As can be seen from
FIG. 6(a), the absorbance of the rhodium solution used in Examples 1
through 4 for the ray having a wavelength of 300 nm is 0.2. By contrast,
as can be seen from FIG. 6(b), the absorbance of the rhodium solution
used in Example 5 for the ray having a wavelength of 300 nm is 1. As seen
from a comparison of Examples 1 through 4 and Example 5 in Table 1, the
degree of dispersion of rhodium can be higher when the absorbance of the
rhodium solution prepared in step S2 for the ray having a wavelength of
300 nm is 0.8 or lower than when the absorbance exceeds 0.8. A
conceivable reason why this occurs is that the state of rhodium ions in
the solution influences the degree of dispersion.

[0090] Now, results of inspection on the relationship between the ratio of
zirconia with respect to the metal oxide support prepared in step S1
(i.e., the ratio of zirconium as being converted into an oxide) and the
NOx purification ratio realized by the resultant catalyst will be
described.

[0091] FIG. 7 shows the relationship between the ratio of zirconia (mol %)
with respect to the metal oxide support and the NOx emission (g/km).
The NOx emission was measured by use of a motorcycle having a
displacement of 125 cc under the EU3 exhaust gas test conditions. Table 2
shows the chemical compositions of the metal oxide supports used at
points A and B in FIG. 7. Table 3 shows the CO emission, THC emission and
NOx emission at points A and B in FIG. 7.

[0092] It is understood from FIG. 7 that when the ratio of zirconia with
respect to the metal oxide support is in a range of 50 mol % or higher
and 95 mol % or lower, the NOx emission can be decreased and the
NOx purification ratio can be raised. It is also understood from
FIG. 7 that when the ratio of zirconia with respect to the metal oxide
support is in a range of 70 mol % or higher and 90 mol % or lower, the
NOx emission can be further decreased and the NOx purification
ratio can be further raised. For example, comparing the two points A and
B in FIG. 7, the NOx emission is smaller and the CO emission and the
THC emission are also smaller at point B, at which the ratio of zirconia
is 78.1 mol %, than at point A, at which the ratio of zirconia is 54.7
mol %.

[0093] In the description given so far, the pH is adjusted after the metal
oxide support is mixed in the rhodium solution. As shown in FIG. 8, the
pH may be adjusted before the metal oxide support is mixed in the rhodium
solution.

[0094] In the example shown in FIG. 8, first, a metal oxide support
containing zirconium is prepared (step S1). Next, a rhodium solution is
prepared (step S2). Step S1 and step S2 may be performed in any order.

[0095] Next, ammonium carbonate, ammonium hydrogencarbonate or ammonia
water are added to the rhodium solution to adjust the pH of the rhodium
solution to be in a prescribed range (step S3').

[0096] Next, the metal oxide support is mixed in the rhodium solution
(step S4'). The prescribed range (range of pH of the rhodium solution) in
step S3' is set such that the pH of the rhodium solution becomes a value
in a range of 3.0 or higher and 7.5 or lower after step S4'. Namely, the
pH is adjusted in step S3' in consideration that the pH is changed when
the metal oxide support is mixed in the rhodium solution. Typically, the
rhodium solution, after being mixed with the metal oxide support, is
stirred by a stirrer or the like, and then left at a prescribed
temperature (e.g., 60° C.) for a prescribed time duration (e.g., 1
to 5 hours). As a result of step S4', rhodium is adsorbed to the metal
oxide support containing zirconium.

[0097] Next, the rhodium solution is dried and burned (step S5). Then, the
resultant catalyst powder is used to form a catalyst layer on a surface
of a honeycomb-like substrate (step S6).

[0098] As described above, the pH may be adjusted after the metal oxide
support is mixed in the rhodium solution or before the metal oxide
support is mixed in the rhodium solution. Namely, it is merely needed
that after the metal oxide support and the rhodium solution are prepared,
the step of adding the metal oxide support, as well as ammonium
carbonate, ammonium hydrogencarbonate or ammonia water, to the rhodium
solution to obtain the rhodium solution having a pH adjusted to a range
of 3.0 or higher and 7.5 or lower is executed. This step may include
steps S3 and S4 shown in FIG. 1 or step S3' and S4' shown in FIG. 8.
Since the pH of the resultant rhodium solution is in a range of 3.0 or
higher and 7.5 or lower, the degree of adsorption of rhodium to the metal
oxide support containing zirconium can be increased.

[0099] When the pH is adjusted after the metal oxide support is mixed in
the rhodium solution as shown in FIG. 1, there is an advantage that
rhodium can be dispersed more uniformly. When the pH is adjusted before
the metal oxide support is mixed in the rhodium solution, there is an
advantage that a metal oxide support containing a component which is
easily soluble in acid can be used.

[0100] An exhaust gas purifying catalyst produced by a production method
in this embodiment provides a high level of purification performance and
is highly durable, and therefore is preferably usable for various types
of motor vehicles.

[0101] FIG. 9 shows a motorcycle 100 including an exhaust gas purifying
catalyst produced by a production method in this embodiment.

[0102] The motorcycle 100 includes an internal combustion engine 1, an
exhaust pipe 7 connected to an exhaust port of the internal combustion
engine 1, and a muffler 8 connected to the exhaust pipe 7. Exhaust gas
from the internal combustion engine 1 is guided outside by the exhaust
pipe 7. In the exhaust pipe 7, an exhaust gas purifying catalyst produced
by the above-described production method is provided. The motorcycle 100
includes the exhaust gas purifying catalyst which provides a high level
of purification performance and is highly durable, and therefore can
decrease the emission of NOx or the like.

[0103] Herein, the motorcycle is shown as an example. The exhaust gas
purifying catalyst produced by the production method in this embodiment
is preferably usable for all types of vehicles, not only for motorcycles.
For example, the exhaust gas purifying catalyst produced by the
production method in this embodiment is usable for ATVs such as buggies.

INDUSTRIAL APPLICABILITY

[0104] The present invention can provide a method capable of producing an
exhaust gas purifying catalyst including a metal oxide support containing
zirconium and rhodium of a small particle size which is supported on the
metal oxide support at a high degree of dispersion. The exhaust gas
purifying catalyst produced by the production method according to the
present invention is preferably usable for various types of motor
vehicles such as motorcycles and the like.